This invention relates to semiconductor light emitting devices and, more particularly, to semiconductor light emitting diodes (LEDs).
Semiconductor LEDs are widely known solid-state lighting elements that are capable of generating light upon application of voltage thereto. LEDs generally include a diode region including therein an n-type layer, a p-type layer and a p-n junction. An anode contact ohmically contacts the p-type layer and a cathode contact ohmically contacts the n-type layer. The diode region may be epitaxially formed on a substrate, such as a sapphire, silicon, silicon carbide, gallium arsenide, gallium nitride, etc., growth substrate, but the completed device may not include a substrate. The diode region may be fabricated, for example, from silicon carbide, gallium nitride, gallium phosphide, aluminum nitride and/or gallium arsenide-based materials, and/or from organic semiconductor-based materials. Finally, the light radiated by the LED may be in the visible or ultraviolet (UV) regions, and the LED may include wavelength conversion materials such as phosphor.
Some LEDs include top anode and cathode contacts, and are configured for bottom mounting. More specifically, some commercially available LEDs may include a silicon carbide substrate having first and second opposing faces, a diode region on the first face and anode and cathode contacts on the diode region opposite the silicon carbide substrate. An example of these LEDs is the ETC™ family of LEDs manufactured by CREE®. FIG. 1 is a reproduction of a chip diagram from the “Cree® TR2432™ LEDs Data Sheet CxxxTR2432-Sxx00”. As shown in the Die Cross-Section of FIG. 1, these LEDs include a relatively thin diode region and a relatively thick silicon carbide substrate that includes oblique sidewalls to enhance light emission through the sidewalls. The Top View of FIG. 1 shows an anode and a cathode bond pad, and the Bottom View illustrates a bottom surface. As noted at Page 1 of this data sheet, “[T]he design may be suited for industry standard sideview packages as it is die attachable with clear epoxy and has two top contacts, consistent with industry standard packaging”.
Other commercially available LEDs may include a sapphire substrate having first and second opposing faces, a diode region on the first face, and anode and cathode contacts on the diode region opposite the sapphire substrate. One example of these sapphire-based LEDs is the NLX-5 Blue Power Die marketed by BRIDGELUX® and described in a specification sheet entitled “NLX-5 Blue Power Die 38×38 mil BXCA3838XXX-YY-Z”, Specification No. BCDS BXCA3838 020509 Rev. 3. These LEDs include a bottom reflector with thin gold. More specifically, the bottom of the device includes a layer of silicon dioxide (about 0.5 μm thick) on the sapphire substrate, followed by alternating layers of titanium dioxide and silicon dioxide on the layer of silicon dioxide, and followed by layers of aluminum, titanium, chromium, platinum and gold that are stacked on the alternating layers of titanium dioxide and silicon dioxide. Another example of an LED that includes a sapphire substrate is the NS6L083BT LED marketed by Nichia Corporation, and described in the “Specifications for Nichia Chip Type Warm White LED Model: NS6L083BT”, Nichia Specification No. STS-DA1-0181C. In these devices, alternating layers of aluminum and tungsten are provided directly on the sapphire substrate, followed by a layer of platinum and a layer of gold-tin.